nubus: use bitmap to manage available slots

nubus: use bitmap to manage available slots

Convert nubus_device_realize() to use a bitmap to manage available slots to allow
for future Nubus devices to be plugged into arbitrary slots from the command line
using a new qdev "slot" parameter for nubus devices.

Update mac_nubus_bridge_init() to only allow slots 0x9 to 0xe on Macintosh machines
as documented in "Designing Cards and Drivers for the Macintosh Family".

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210924073808.1041-5-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>